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20 October 2012

A "Well-Deserved" Ice-Cream after Exercise

I often hear people say that after strenuous exercise they deserve a treat, an ice-cream or some kind of "forbidden" food. They think that after running off some calories, that little sweet treat won't do any harm.

The reality, or the biochemical reaction in our bodies, is somewhat different.
There is an enzyme called lipoprotein lipase (LPL) involved in process of fat accumulation. Let me explain, and here I will talk about the difference between two kind of tissues in our bodies: muscle tissue and fat tissue. Both of these tissues use fatty acids (calories). Our muscle tissue burns it when we exercise, the fat tissue stores these fatty acids away for later use. LPL is needed to make the use of fatty acid possible. If LPL activity is high in our muscle tissue, we burn more fatty acids (calories), and if LPL activity is high in our fat tissue, we will store more fat. In other words, if LPL activity is high in a particular cell, the more fatty acids it will absorb.

Now let's see what influences LPL activity in our muscle or fat tissues.

Hormones
Insulin (it is a hormone!) is the primary regulator of LPL activity. It will increase LPL activity in fat tissue (high insulin levels mean more fat storage), and decrease LPL activity in muscle tissue (less fatty acids available for the muscle to burn).

"When insulin levels drop [on a low-carbohydrate diet], the LPL activity on the fat cells decreases and the LPL activity on the muscle cells increases - the fat cells release fatty acids, and the muscle cells take them up and burn them."
- Gary Taubes

In other words, if our insulin level is low, our muscle have more fatty acids available and our fat tissue will not be able to store these fatty acids.

Sex hormones also have an influence on LPL activity. For instance, estrogen (a female sex hormone) increases LPL activity in the muscle cells and decreases it in the fat cells. This explains why, after menopause, women tend to gain weight. This increase in weight is not due to overeating, it is caused by low estrogen levels.

Diets restricted in calories
I'm sure you've already heard people complain that they could not keep the weight off after a low-calorie diet. When people start to eat normally after a few weeks on salads and diet-water, they tend to weigh more than at the beginning of the diet. The reason is that a calorie-restricted diet will decrease LPL activity in the muscle cells, and increase it in the fat cells. From an evolutionary point of view, this makes sense: when food is not available, our hormones tell our body to decrease muscle activity and to store fat in our adipose tissues.

"Rats [...] lay down more fat, Greenwood reported, if they are kept to a strict diet than they will if they are allowed to eat freely to satisfy their hunger. The less they are allowed to eat, however, the smaller their muscles will be; their brains and kidneys will also be 'significantly reduced' in size."
- Gary Taubes

This means that if we eat less, we will deposit fat, and our daily activities will be fueled by the "decomposition" of our muscles and organs. As a result, on the long run, we will have smaller muscles and organs, but our fat tissue will remain the same.

Exercise
During exercise, our muscles burn fatty acids (calories), so the LPL activity is increased in our muscle tissue and decreases in our fat tissue. What happens after exercise? When the exercise is over, the LPL activity in the fat tissue increases, it means that we are, once again ready to store the calories in our fat tissue.